Image sensor with SOI substrate

ABSTRACT

An imager pixel utilizing a silicon-on-insulator substrate, a photodiode in said substrate below the buried oxide, and a dual contact to said photodiode and methods of forming said imager pixel. The photodiode has an increased fill factor due to its increased size relative to the pixel.

This application is a divisional of U.S. patent application Ser. No.11/178,799, filed Jul. 12, 2005, now U.S. Pat. No. 7,271,025 theentirety of which is incorporated by reference.

FIELD OF THE INVENTION

The invention relates to imager technology. In particular, the inventionrelates to imager devices with an SOI substrate.

BACKGROUND OF THE INVENTION

CMOS imagers are known in the art. Exemplary CMOS imaging circuits,processing steps thereof, and detailed descriptions of the functions ofvarious CMOS elements of an imaging circuit are described, for example,in U.S. Pat. No. 6,140,630, U.S. Pat. No 6,376,868, U.S. Pat. No6,310,366, U.S. Pat. No 6,326,652, U.S. Pat. No 6,204,524, and U.S. Pat.No 6,333,205, each assigned to Micron Technology, Inc. The disclosuresof the forgoing patents are hereby incorporated by reference in theirentirety.

A top-down and cross-sectional view of a typical and exemplary CMOSfour-transistor (4T) pixel cell 10 is illustrated in FIGS. 1 a and 1 b.As it will be described below, the CMOS sensor pixel 10 includes aphotoconversion device 12 (e.g., photo-generated charge accumulatingarea) in a portion of the substrate 14. This area 12 can be a pinnedphotodiode (FIG. 1 b). The pinned photodiode is termed “pinned” becausethe potential in the photodiode is pinned to a constant value when thephotodiode is fully depleted. It should be understood, however, that thepixel 10 may also typically include a photogate or other image to chargeconverting device, in lieu of a pinned photodiode, as the initialaccumulating area 12 for a photoconversion device.

The pixel 10 of FIG. 1 a has a transfer gate 16 for transferringphotoelectric charges generated in the charge accumulating region 12 toa floating diffusion region (sensing node) 18. The floating diffusionregion 18 is connected 24 to a gate 22 of a source follower transistor.The source follower transistor provides an output signal to a row selectaccess transistor having gate 26 for selectively gating the outputsignal to terminal 28. A reset transistor having gate 20 resets thefloating diffusion region 18 from a supply voltage applied at asource/drain region 30 between gates 20 and 22 to a specified chargelevel before each charge transfer from the charge accumulating region12. The pixel 10 is typically isolated from other like cells of animager array by shallow trench isolation regions 32. As shown in FIG. 1a, the charge accumulation region 12 of the pixel 10 is somewhat limitedin area by the area taken up by the associated pixel circuitry.

A cross-sectional view of the exemplary pixel 10 of FIG. 1 a taken alongline a-a′ is illustrated in FIG. 1 b. The charge accumulating region 12is formed as a pinned photodiode, which has a photosensitive region orp/n/p junction formed by a p-type layer 34, an n-type region 36 and theunderlying p-type substrate 14. The pinned photodiode includes twop-type regions 14, 34 so that the n-type photodiode region 36 is fullydepleted at a pinning voltage. The floating diffusion region 18 adjacentthe transfer gate 16 and the source/drain region 30 are preferablyn-type and are formed in a p-well 40 in the substrate 14. Also shown inFIG. 1 b is contact 24, which connects the floating diffusion region 18with the source follower transistor gate 22 (FIG. 1 a at 24) and contact38, which connects the source/drain region 30 with a voltage supply.

Generally, in CMOS image sensors such as the pixel 10 of FIGS. 1 a and 1b, incident light causes electrons to collect in region 36. A maximumoutput signal, which is produced by the source follower transistorhaving gate 22, is proportional to the number of electrons to beextracted from the region 36. The maximum output signal increases withincreased electron capacitance or acceptability of the region 36 toacquire electrons. The electron capacity of pinned photodiodes typicallydepends on the doping level of the image sensor and the dopantsimplanted into the active layer.

CMOS imagers typically suffer from poor signal to noise ratios and poordynamic range as a result of the inability to fully collect and storethe electric charge collected in the region 36. Since the size of thepixel electrical signal is very small due to the collection of photonsin the photo array, it would be desirable for the signal to noise ratioand dynamic range of the pixel to be as high as possible. Additionally,as semiconductor devices are scaled down, more efficient pixelconfigurations are desirable to maximize photoconversion device size andminimize the area required for the related pixel circuitry.

Silicon-on-insulator (SOI) substrates are increasingly being used inhigh performance CMOS integrated circuits. For system on chip (SOC)applications that demand a significant amount of logic circuitry, SOIcan offer benefits in power efficiency, low parasitic capacitance, andhigh switching speeds. In SOI technology, a layer of SiO₂ (or Si₃N₄),referred to as the buried oxide (BOX), is beneath an active siliconlayer, formed either through a high dose oxygen implantation andsubsequent annealing (Separation by Implantation of Oxygen, or SIMOX),or by bonding two oxidized silicon wafers together (BESOI) and thenetching-back one of those wafers until a thin layer of silicon remainsabove the oxide layer where the two wafers had been joined. A CMOSprocess is typically implemented on these wafers, in which theinsulating BOX layer drastically reduces leakage currents, eliminatesthe problem of latchup (parasitic bipolar action as a result of then/p/n and p/n/p junctions formed by the various implants used for theCMOS process), and the reduction of RF parasitics, since the devices nowsit on an insulating layer. Image sensors could benefit from suchadvantages over typical substrates; however, integrating an image sensorarray into SOI technology has proven difficult.

BRIEF SUMMARY OF THE INVENTION

An exemplary embodiment of the invention includes a CMOS imager (i.e.,image sensor) pixel that utilizes a silicon-on-insulator, i.e., SOI,substrate and a photosensor (e.g., pinned photodiode) that substantiallyspans the pixel area of the substrate, which provides for a large fillfactor. A dual contact can be utilized to connect the photodiode withthe pixel circuitry above. The substrate surrounding the dual contactcan be doped p-type to reduce dark current.

In an exemplary embodiment of the invention, the buried oxide of the SOIsubstrate can be doped to provide various characteristics for the pixel.

The substrate of the pixel below the buried oxide can comprise an epilayer to provide epi substrate benefits for the imager. Optionally, theepi layering can be a graded. The epi portion of the substrate can alsoinclude complimentary layers.

In an exemplary embodiment of the invention, the pixel is isolated fromother like pixels of an array by STI. In an alternative embodiment, thepixel is isolated from other pixels by a diffused isolation region.

In an exemplary embodiment of the invention, the pixel senses light fromthe side of the wafer where the pixel circuitry is positioned, i.e., thefront side. In an alternative embodiment, the pixel senses light fromthe opposite side of the wafer, i.e., the back side.

In an exemplary embodiment of the invention, the pixel comprisesmultiple photodiodes to sense multiple colors of light. Thesephotodiodes are stacked within the substrate below the BOX layer. Two ormore such photodiodes can be utilized.

These and other features of the invention will be better understood fromthe following detailed description, which is provided in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows an overhead view of a typical prior art CMOS imagerpixel.

FIG. 1 b shows a cross-section view of a pixel as shown in FIG. 1through line a-a′.

FIG. 2 shows a pixel in accordance with an embodiment of the invention.

FIG. 3 shows the pixel of FIG. 2 in cross-section through line b-b′.

FIGS. 4-8 show a pixel, as shown in FIG. 2, at various stages offabrication, in accordance with the invention.

FIG. 9 shows a pixel for sensing multiple colors in accordance with anembodiment of the invention.

FIG. 10 shows the pixel of FIG. 9 in cross-section through line c-c′.

FIG. 11 shows an imager constructed in accordance with an embodiment ofthe invention.

FIG. 12 shows a processor system incorporating at least one imagerconstructed in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although this invention will be described in terms of certain exemplaryembodiments, other embodiments will be apparent to those of ordinaryskill in the art, which also are within the scope of this invention.Accordingly, the scope of the invention is defined only by reference tothe appended claims.

The terms “substrate” or “wafer” are used interchangeably in thefollowing description. A silicon-on-insulator (SOI) substrate is used inaccordance with the invention, which may also include epitaxial layersof silicon supported by a base semiconductor foundation. When referenceis made to a substrate or wafer in the following description, previousprocess steps may have been utilized to form regions or junctions in orover a base semiconductor or foundation.

The term “pixel” refers to a photo-element unit cell containing a chargeaccumulating photoconversion device and associated transistors forconverting electromagnetic radiation to an electrical signal. The pixelsdiscussed herein are illustrated and described as 4T (4 transistors)CMOS pixel circuits for the sake of example only. It should beunderstood that the invention is not limited to a four transistor (4T)pixel or CMOS technology, but may be used with other pixel arrangementshaving fewer (e.g., 3T) or more (e.g., 5T) than four transistors andother imager technology. Although the invention is described herein withreference to the architecture and fabrication of one pixel, it should beunderstood that this is representative of a plurality of pixels astypically would be arranged in an imager array having pixels in rows andcolumns. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined only by the appended claims.

The invention will now be explained with reference to the accompanyingfigures wherein like reference numbers are used consistently for likefeatures throughout the drawings. FIG. 2 is a top-down view of anexemplary CMOS pixel 100 constructed in accordance with the invention,while FIG. 3 is a cross-sectional view of pixel 100 along line b-b′. Thepixel 100 has a pinned photodiode 101 as a charge accumulationphotoconversion device. The photodiode 101 is represented in FIGS. 2 and3 by its n-type region 118, which covers an area of the substratecorresponding to substantially the entire pixel 100 area. The photodiode101 can occupy up to 95% or more of the total pixel area, relative tothe substrate. This large photodiode 101 area, relative to theassociated pixel circuitry, provides a large fill factor for the pixel100.

The pixel circuitry, supported by a thin (typically about 1,000 Å toabout 1,500 Å thick) silicon layer 113 over a buried oxide (BOX) 112(FIG. 3), which can be about 2,000 Å to about 3,000 Å thick, includes atransfer transistor having gate 136, a reset transistor having gate 138,a source follower transistor having gate 140, and a row selecttransistor having gate 142. Associated with these gates (136, 138, 140,and 142) are source/drain active areas 146, one of which is a floatingdiffusion region 146 a.

The charge collection n-type region 118 of the photodiode 101 iselectrically connected with the transfer transistor (having gate 136) bycontact 162. Contact 162, which can be tungsten, polysilicon, or otherconductive material, is configured to penetrate through the source/drainregion 146 adjacent the transfer transistor gate 136 to the SOIsubstrate 110 beneath the BOX layer 112 (FIG. 3). The contact 162 can besurrounded by a doped p-type region 158 of the substrate 110, whichretains the pinned characteristics of the photodiode 101 and mitigatesdark current in the pixel 100, but does not interfere with the chargetransfer from the photodiode 101 to the transfer transistor. Contact 162is connected by a conductive strap 166 to a second contact 164, whichalso penetrates the SOI substrate, but outside the photodiode 101 regionthrough the STI (or diffused isolation) region 124. The conductive strap166 and contacts 162 and 164 connect the substrate 110 and the chargeaccumulating n-type region 118 of the photodiode (FIG. 3), whichprovides the pinning effect for the photodiode 101; the electricalconnection between the dual contacts 162 and 164 ensures that the sameelectrical potential exists in both the photodiode 101 portion of thesubstrate 110 and in the p-well portion of the substrate 110. A p-typedoped region 158 of the substrate 110 can also surround contact 164.

Also shown in FIGS. 2 and 3 are contacts to the pixel transistors. Thesource/drain region between the transfer transistor gate 136 and thereset transistor gate 138 is considered a floating diffusion region 146a of the pixel 100 and has contact 168 and interconnect line 170connected thereto, which electrically connects the floating diffusionregion 146 a to the source follower transistor gate 140. Between thereset transistor gate 138 and the source follower gate 140 is a contact172, which connects the floating diffusion region 146 a of pixel 100 toa voltage source (e.g., V_(aa)) for resetting the floating diffusionregion 146 a to a reset state. Connected to the source/drain region 146adjacent the row select transistor gate 142 is contact 176, whichconnects the output of the row select transistor to a column signalread-out circuit.

Referring again to FIG. 3, the substrate 110 is semiconductor based(e.g., silicon) and can optionally be p-on-p+ epi material (a suitableoption for a typical p/n/p photodiode), where the optional p+ epi layer110 a is formed over the underlying substrate 110, and the p+ epi can bea graded-epi layer. Alternatively, a complimentary epi substrate havingn-on-n+ epi (suitable for a n+/p/n photodiode) can be used, havingoptional n+ epi layer 110 a, which can also be graded. Epi (epitaxial)substrates are beneficial here because they provide lower carrierlifetime for electrons, which are minority carriers in the p+ region,which reduces cross talk between pixels.

The lower substrate 110 can typically have p-type conductivity, makingit a part of the photodiode 101. FIG. 3 shows the n-type region 118 ofthe photodiode 101, which defines the area of the photodiode 101, whichis shown in FIG. 2 as spanning substantially the entire pixel 100 area.Over the n-type region 118 is a p-type layer 122, which can be providedby making the BOX 112 boron-rich or otherwise formed by dopant (e.g.,boron) implantation below the BOX 112. Over the BOX 112 is the activearea of the wafer, which can be a thin silicon film 113 about 500 Å toabout 5,000 Å thick, preferably about 1,000 Å to about 1,500 Å thick.The active area of each pixel of the array, of which the pixel 100 is apart, can be isolated by STI regions 124. Alternatively, diffusedisolation (STI-free) can be used for isolating pixels, which mitigatesdark current. A diffused isolation region (area 124) would occupy someof the wafer volume occupied by the STI region 124 in FIG. 3. Thediffused isolation region (i.e., 124) is an area of the wafer heavilydoped with p-type dopant to prevent current leakage between pixels.

The BOX 112 can be made boron-rich so as to provide a p-type layer 122over the n-type region 118 for the photodiode 101 by the diffusion outof the p-type dopant into the underlying substrate 110, as indicated inthe preceding paragraph. Alternatively, the BOX 112 can bedeuterium-rich (by implantation), which provides passivation of thesemiconductor interface, since deuterium will passivate by ensuring thatany dangling silicon bonds do not contribute to the electrical chargestates in the pixel. Deuterium is a superior option to hydrogen in thisregard since it is a heavier isotope of hydrogen. Alternatively, the BOX112 can be fluorine-rich; also for passivation.

The thin silicon film layer 113 over the BOX 112 supports thetransistors (e.g., transfer, reset, source follower, and row select) ofthe pixel 100. These transistors have associated transfer transistorgate 136, reset transistor gate 138, source follower transistor gate140, and row select transistor gate 142. Between these gates (136, 138,140, and 142) are n-type source/drain active regions 146 (includingfloating diffusion region 146 a), which can be fully depleted(source/drain regions 146 extend to the BOX 112), as shown, or partiallydepleted (source/drain regions 146 extend partially to the BOX 112.Between the source/drain regions 146 and under the gates (136, 138, 140,142) are channel regions 148 for the transistors. A thin insulatorspacer material 150 can be layered over the substrate for protection andelectrical isolation of the transistor gates. 100. As anotheralternative, region 124 can also be formed by LOCOS. An implant 126 isperformed to form a p-well 128 in the silicon layer 113 (over the BOX112).

STI region 124 and p-well 128 formation is followed by conventionaltransistor formation over the silicon layer 113, as shown in FIG. 7. Anoxide layer 130 is formed over the wafer; this will be the gate oxidesfor the resulting transistors. This is followed by formation of aconductive material layer 132, e.g., doped poly or tungsten, which willbe the gate electrodes. An insulative material layer 134, such asnitride, is formed over the conductive material layer 132 to serve as aninsulative and protective cap for the transistor gates. These layers,130, 132, 134, are then patterned, e.g., with photoresist, and etched toleave gate stacks 136 (transfer transistor gate), 138 (reset transistorgate), 140 (source follower transistor gate), and 142 (row selecttransistor gate). After defining the gates (136, 138, 140, 142)source/drain regions 146 are formed by n-type dopant implant 144. Aninsulative film is then formed over the gates (136, 138, 140, 142) andwafer and etched back to leave an insulative layer and sidewalls 150over the wafer and on the sides of the gates (136, 138, 140, 142).Periphery devices can also be formed at this time.

After forming the transfer transistor gate 136, the reset transistorgate 138, the source follower transistor gate 140, and the row selecttransistor gate 142, a protective layer 160 is formed over the wafer andplanaraized, e.g., by CMP. Then, dual vias 154 are formed in thesubstrate 110, as shown in FIG. 8. The vias 154 are formed by patterningthe wafer with photoresist 152 and anisotropically etching to the n-typeregion 118 of the photodiode 101 and to an adjacent region of the waferbelow the STI (or, e.g., diffused isolation) region 124. Vias (notshown) for contacts to the pixel circuitry can also be formed at thistime. An angled implant 156 of p-type dopant, e.g.,

FIG. 3 shows the contacts 162 and 164 to the photodiode of the pixel100. As shown, contact 162 fills a via 154 and penetrates the substrateat the source/drain region 146 of the transfer transistor (gate 136),through the BOX 112, through the p-type layer 122 of the photodiode, andterminating at the n-type region 118 of the photodiode. The contact 162hooks the substrate to the photodiode 101, making it pinned, by itsconnection with the second contact 164 through the conductive strap 166.The second contact 164 also penetrates the substrate and BOX 112, butthrough the STI 124 (or other isolation feature, e.g., diffusedisolation region or field oxide formed by LOCOS). These contacts 162 and164 are surrounded by p-type doped regions 158, which can be formed byboron implantation.

Over the transistor gates (136, 138, 140, 142), FIG. 3 shows atransparent and dielectric protective layer 160. Protective layer 160may be a layer of BPSG, PSG, BSG, silicon dioxide, silicon nitride,polyimide, or other well-known light transmissive insulators. Contacts168, 172, and 176 to the transistors are formed in vias in thisprotective layer 160 to the floating diffusion region 146 a andsource/drain regions 146 of the transistors. Interconnects 170 (betweenthe floating diffusion region 146 a and the source follower transistorgate 138), 174 (between the reset transistor and a voltage source), and178 (from the row select transistor to read-out circuitry) are providedover the protective layer 160 in electrical communication with thecontacts 168, 172, and 176, respectively. Over these interconnect layerscan be formed additional light transmissive protective layers and lenses(not shown), as is known in the art.

Although the pixel 100 has been primarily described above such that inoperation it receives light from the side of the wafer above the pixelcircuitry, this is not necessarily required and it is possible to adjustthe positioning of the photodiode 101 so that the pixel 100 receives andsenses light from the opposite side of the wafer. Therefore, in analternative embodiment, the n-type region 118 is spaced deeper in thesubstrate 110 so that it is at a suitable depth to sense light from theside of the wafer opposite the BOX 112 and silicon layer 113.

The pixel 100 can be fabricated as one of an array of like pixels bysteps as shown in FIGS. 4-8. As shown in FIG. 4, processing can begin byproviding a semiconductor wafer 110, which is preferably silicon. A BOX112 (buried oxide) is formed by SIMOX (separation by implantation ofoxygen). Oxygen (or nitrogen) is implanted 114 into the substrate 110(typically about 10¹⁵ to about 10¹⁸ ions cm⁻²) at a depth of about 500 Åor less to about 5,000 Å or more (depth of the BOX 112 determinesthickness of overlying silicon) and at an elevated temperature(typically about 400° C. to about 600° C.). An annealing process(typically about 900° C. to about 1,400° C.), which can be carried outin a high temperature furnace, converts the implanted oxygen intosilicon oxide (or nitrogen into silicon nitride) and thereby forms theBOX 112, which can typically be about 2,000 Å to about 3,000 Å thick. Ifit is desired that the BOX 112 be rich in some element, e.g., boron,fluorine, or deuterium, to achieve certain characteristics in theresulting pixel, it is implanted 116 at the proper depth. As indicatedabove, a boron-rich BOX 112 will provide a p-type layer 122 for thephotodiode; a deuterium or fluorine-rich BOX 112 will aid in passivationof the semiconductor wafer.

As shown in FIG. 5, after formation of the BOX 112, implantation 120 ofan n-type dopant, e.g., phosphorus, creates an n-type region 118 belowthe BOX, which will serve as the primary charge collection body of thephotodiode 101 of the pixel cell 100. For a pixel 100 in accordance withthe invention, having a BOX 112 thickness of about 2,000 Å to about3,000 Å and a silicon layer 113 thickness of about 1,000 Å to about1,500 Å, an implant energy in the range of about 500 keV to about 3 MeVcan be used. The substrate 110 is masked with, e.g., photoresist (notshown), for implantation 120; the photoresist is then removed. Unlikeconventional pixel designs (FIGS. 1 a and 1 b) and even other pixeldesigns utilizing SOI technology, here the n-type region 118 of thephotodiode 101 substantially (or totally) spans (up to 95% or more) theentire area of the pixel 100. This large photodiode 101 area providesfor an increased fill factor for the pixel 100. If the BOX 112 isboron-rich, a p-type layer 122 is formed over the n-type region 118 bythe diffusion of boron from the BOX 112 into the substrate between then-type region 118 and the BOX 112. If the BOX 112 is not boron-rich, animplantation step may be employed, e.g., using boron, to create thep-type layer 122 below the BOX 112.

As shown in FIG. 6, after formation of the BOX 112 and photodioderegions (n-type 118 and p-type 122), STI (shallow trench isolation)regions 124 are formed in the silicon layer 113 over the BOX 112 andsurrounding the portion of the wafer dedicated for the pixel 100. TheSTI (or other isolation) regions 124 isolate the pixel 100 from otherlike cells. If STI regions 124 are used, they are formed by etching thesilicon layer 113 to the BOX 112. After etching, a boron implant can beutilized to p-type dope the sidewalls of the STI regions 124. The etchedregions of the substrate are filled with an insulating material, such assilicon oxide. Removal of excess insulating material and waferplanarization can be accomplished here by, e.g., CMP. As an alternative,region 124 can be a diffused isolation region formed by concentratedimplantation of p-type dopant around the pixel 100. A diffused isolationregion mitigates dark current in the pixel boron, is performed to formdoped sidewalls 158 of the vias 154. This is followed by formingcontacts 162 and 164 (as well as 168, 172, and 176, FIG. 3) bydeposition, e.g., by sputtering, and defining, e.g., etching-back orCMP. The conductive strap 166 and interconnect lines 170, 174, and 178are formed, e.g., by sputtering, patterning and etching, resulting inthe pixel cell 100 as shown in FIG. 3. As discussed above with referenceto FIGS. 2 and 3, contacts 162 and 164 can be a variety of conductivematerials, such as aluminum, tungsten, titanium, and doped poly. Theconductive strap 166 and interconnect lines 170, 174, and 178 can alsobe a variety of conducive materials, such as tungsten, copper, gold,silver, aluminum and doped poly.

An alternative embodiment of the invention is shown in FIG. 9, whichshows a pixel 300 similar to pixel 100 shown in FIG. 2 and 3; however,pixel 300 is a multicolor pixel having at least two photodiodes (e.g.,101 and 301) for sensing at least two light wavelength ranges, e.g., redand blue. Pixel 300 is shown having three photodiodes (i.e., 101, 201,and 301), represented by their n-type regions 118, 218, and 318. Thethree photodiodes 101, 201, and 301 of pixel 300 can be used to senseblue, green, and red light. FIG. 9 shows an exemplary layout for thecircuitry for each of these photodiodes 101, 201, and 301. As with pixel100 of FIG. 2, the photodiodes 101, 201, and 301 of pixel 300substantially span the entire pixel area across the wafer, providing alarge fill factor.

The first photodiode 101, having n-type region 118 and p-type layer 122(FIG. 10), is associated with circuitry like pixel 100 of FIG. 2, asshown by the like reference numbers. The second photodiode 201, havingn-type region 218 and p-type layer 222 (FIG. 10), is connected bycontact 262 to transfer transistor gate 236 by the proximatesource/drain region 246.

Also associated as circuitry for the second photodiode 201 is a resettransistor gate 238, a source follower transistor gate 240, and a rowselect transistor gate 242, and proximate source drain regions 246. Thesecond photodiode's 201 floating diffusion region 246 a has contact 268connected by interconnect line 270 with the source follower transistorgate 240. Between the reset transistor gate 238 and the source followertransistor gate 240 is contact 270, which is connected with a voltagesource (e.g., V_(aa)). Proximate the row select transistor gate 276 iscontact 276, which is connected to signal readout circuitry.

The third photodiode 301, having n-type region 318 and p-type layer 322(FIG. 10), is connected by contact 362 to transfer transistor gate 336by the proximate source/drain region 346.

Associated as circuitry for the third photodiode 301 is a resettransistor gate 338, a source follower transistor gate 340, and a rowselect transistor gate 342, and proximate source drain regions 346. Thethird photodiode's 301 floating diffusion region 346 a has contact 368connected by interconnect line 370 with the source follower transistorgate 340. Between the reset transistor gate 338 and the source followertransistor gate 340 is contact 370, which is connected with a voltagesource (e.g., V_(aa)). Proximate the row select transistor gate 376 iscontact 376, which is connected to signal readout circuitry.

FIG. 10 shows the pixel 300 through cross-section c-c′ and illustrateshow the three photodiodes 101, 201, and 301 can be positioned withrespect to each other and the pixel circuitry. As shown, photodiode 201(having n-type region 218 and p-type region 222) is between photodiode101 (having n-type region 118 and p-type region 122) and photodiode 301(having n-type region 318 and p-type region 322). There need notnecessarily be three photodiodes, but there must be at least two in thisembodiment. If there are only two photodiodes (e.g., 101 and 301), theycan be positioned in the substrate 110 so that, for example, one sensesblue light at a substrate depth of about 0.5 μm to about 1.0 μm and onesenses red light at a substrate depth of about 2.0 μm to about 5.0 μm orgreater; a second pixel (not shown) can then be utilized to sense greenlight at a substrate depth of about 1.5 μm. Other color sensingphotodiode combinations (e.g., blue/green, red/green, etc.) are alsopossible. If three photodiodes 101, 201, and 301 are utilized, as shownin FIG. 10, they are stacked in the substrate 110 at the appropriatedepths such that the n-type region 218 is positioned for sensing greenlight and one of the n-type regions 118 and 318 is positioned forsensing red light and the other is positioned for sensing blue light(with respect to the side of the wafer where light impacts the wafer,e.g., front-side or back-side).

The photodiodes 101, 201, and 301 of pixel 300 are connected to theirassociated circuitry in the same manner as that of pixel 100 in FIG. 3.Contact 262 (shown in hatching as it is not necessarily in the sameplane as the rest of FIG. 10) extends lower in the substrate 110 thancontact 162 and is formed to connect n-type region 218 with thesource/drain region 246 (FIG. 9) of the transfer transistor gate 236.Contact 262 can be surrounded by a p-type region 258 of the substrate tomitigate dark current. Contact 362 (shown in less dense hatching thancontact 262; also not necessarily in the same plane as the rest of FIG.10) extends lower in the substrate 110 than contacts 162 and 262 and isformed to connect n-type region 318 with the source/drain region 346(FIG. 9) of the transfer transistor gate 336. Contact 362 can besurrounded by a p-type region 358 of the substrate to mitigate darkcurrent. Like contact 162, contacts 262 and 362 are dual contacts andare connected (FIG. 9) to conductive straps 266 and 366, respectively,and contacts 264 and 364, respectively.

FIG. 11 illustrates an exemplary imager 700 (i.e., image sensor) thatmay utilize any embodiment of the invention. The imager 700 has a pixelarray 705 comprising pixels constructed as described above with respectto FIGS. 2, 3, 9 and 10 or using other pixel architectures. Row linesare selectively activated by a row driver 710 in response to row addressdecoder 720. A column driver 760 and column address decoder 770 are alsoincluded in the imager 700. The imager 700 is operated by the timing andcontrol circuit 750, which controls the address decoders 720, 770. Thecontrol circuit 750 also controls the row and column driver circuitry710, 760 in accordance with the invention.

A sample and hold circuit 761 associated with the column driver 760reads a pixel reset signal V_(rst) and a pixel image signal V_(sig) forselected pixels. A differential signal (V_(rst)-V_(sig)) is amplified bydifferential amplifier 762 for each pixel and is digitized byanalog-to-digital converter 775 (ADC). The analog-to-digital converter775 supplies the digitized pixel signals to an image processor 780 whichforms a digital image. The image processor 780 may also determine thegain setting of the imager 700, which can be used to set the level ofthe voltage applied to the pixels transfer transistor gates.

FIG. 12 shows a processor system 1000, which is a typical processorsystem modified to include an imaging device 1008 (such as an imagingdevice with a pixel as illustrated in FIGS. 2, 3, 9, and 10) of theinvention. The processor system 1000 is exemplary of a system havingdigital circuits that could include image sensor devices. Without beinglimiting, such a system could include a computer system, camera system,scanner, machine vision, vehicle navigation, video phone, surveillancesystem, auto focus system, star tracker system, motion detection system,image stabilization system, and data compression system, and othersystems employing an imager.

System 1000, for example a camera system, generally comprises a centralprocessing unit (CPU) 1002, such as a microprocessor, that communicateswith an input/output (I/O) device 1006 over a bus 1020. Imaging device1008 also communicates with the CPU 1002 over the bus 1020. Theprocessor-based system 1000 also includes random access memory (RAM)1004, and can include removable memory 1014, such as flash memory, whichalso communicate with the CPU 1002 over the bus 1020. The imaging device1008 may be combined with a processor, such as a CPU, digital signalprocessor, or microprocessor, with or without memory storage on a singleintegrated circuit or on a different chip than the processor.

Various embodiments of the invention have been described above. Althoughthis invention has been described with reference to these specificembodiments, the descriptions are intended to be illustrative of theinvention and are not intended to be limiting. Various modifications andapplications may occur to those skilled in the art without departingfrom the spirit and scope of the invention as defined in the appendedclaims.

1. An imager pixel, comprising: a silicon-on-insulator substratecomprising a lower substrate portion, a buried oxide over the lowersubstrate portion, and a silicon layer over the buried oxide; aphotodiode in the lower substrate portion, wherein the pixel has an areadimension with respect to the substrate, and the photodiodesubstantially spans the entire pixel area dimension; a dual contactcomprising a first contact plug to the photodiode and a second contactplug extending to the lower substrate portion adjacent the photodiode;and a conductive strap connecting the first contact plug with the secondcontact plug.
 2. The imager pixel of claim 1, wherein the photodiodespans at least 95% of the pixel area dimension.
 3. The imager pixel ofclaim 1, wherein the dual contact extends through the buried oxide layerand the silicon layer over the buried oxide layer.
 4. The imager pixelof claim 3, wherein the second contact plug extends through an isolationregion to reach the lower substrate.
 5. The imager pixel of claim 4,wherein the first contact plug extends through a source/drain region ofa pixel transistor.
 6. The imager pixel of claim 3, wherein thesubstrate has a p-type conductivity region around the dual contact. 7.The imager pixel of claim 1, wherein the buried oxide is rich in atleast one of boron, fluorine, and deuterium.
 8. The imager pixel ofclaim 1, wherein the lower substrate portion is an n-on-n+ epitaxialmaterial.
 9. The imager pixel of claim 8, wherein a lower substrateportion is a graded n+ epi layer.
 10. The imager pixel of claim 1,wherein the photodiode is configured to sense light impacting the lowersubstrate portion on a side thereof opposite the buried oxide andsilicon layer.
 11. The imager pixel of claim 1, further comprising atleast one second photodiode, wherein each of the photodiode and thesecond photodiode comprises a respective charge collection region, therespective charge collection regions being positioned at respectivedepths within the substrate relative to a light source such that thephotodiode senses a first color of light and the second photodiodesenses a second color of light.
 12. The imager pixel of claim 1, furthercomprising at least one second photodiode and at least one thirdphotodiode, wherein each of the photodiode, the second photodiode, andthe third photodiode each comprises a respective charge collectionregion, the respective charge collection regions being positioned atrespective depths within the substrate relative to a light source suchthat the photodiode senses a first color of light, the second photodiodesenses a second color of light, and the third photodiode senses a thirdcolor of light.
 13. A CMOS imager, comprising: a silicon-on-insulatorsubstrate comprising a lower substrate portion, a buried oxide over thelower substrate portion, and a silicon layer over the buried oxide; anisolation region in the silicon layer, the isolation region defining apixel area; a pinned photodiode in the lower substrate portion; a pixelcircuit comprising a plurality of transistors supported by the siliconlayer; a dual contact from a transfer transistor to a charge collectionregion of the photodiode, the dual contact having a first contactpenetrating the lower substrate portion to the charge collection regionand a second contact penetrating the isolation region and lowersubstrate portion adjacent the photodiode; and a conductive strapconnecting the first contact and the second contact of the dual contact.14. The CMOS imager of claim 13, wherein the first contact extendsthrough a source/drain region of the transfer transistor and the buriedoxide to the charge collection region.
 15. The CMOS imager of claim 13,wherein the buried oxide is rich in at least one of boron, fluorine, anddeuterium.
 16. The CMOS imager of claim 13, wherein the photodiode isconfigured to sense light impacting the lower substrate portion on aside of the lower substrate portion opposite the buried oxide andsilicon layer.
 17. A processor system, comprising: an imager coupled toa processor, the imager comprising an array of pixels, each pixelcomprising: a silicon-on-insulator substrate comprising a lowersubstrate portion, a buried oxide over the lower substrate portion, anda silicon layer over the buried oxide; and a pinned photodiode in thelower substrate portion, wherein the pixel has an area dimension withrespect to the substrate and the photodiode substantially spans theentire pixel area dimension; a first contact through the silicon layerand buried oxide to the pinned photodiode; a second contact through thesilicon layer and buried oxide to the lower substrate portion outsidethe pinned photodiode; and a conductive strap connecting the firstcontact with the second contact.
 18. The processor system of claim 17,further comprising a dual contact to the photodiode, wherein a firstcontact plug extends to the photodiode and a second contact plug extendsto a region of the substrate adjacent the photodiode.
 19. The processorsystem of claim 17, further comprising a shallow trench isolation regionaround the pixel.
 20. The processor system of claim 17, furthercomprising a diffused isolation region around the pixel.